They stock things they make more profit on. If the margins on sugar water are much higher, then they don’t need to sell as much to make it worth stocking it instead of juice. If the margins are higher because consumers are unaware they’re being sold a cheaper-to-manufacture product for the same price because the packaging is deceptive to anyone who hasn’t been told they have to look or is in too much of a rush to have time to look, then shops end up full of sugar water that few consumers actually want.